One of the high school band drumline I use to teach in the past, used a Battery Supply /AC Inverter, Called a Far Outlet
Check out the link
http://www.zzounds.com/item--GXYFAROUTLET We were tried of getting to a band day or an away game, the other band would run power to us and the breaker wouldn't hold up, so half the time our electronic keyboard and electric guitars and sound system wouldn't work for the whole show, so the band director got a Far Outlet, the power was very steady.
We tried a boat battery and a cheap inverter from Auto Zone at first but the power wasn't steady enough for the very nice electronic keyboard and the sound modules. Even with a power stabilzer there was a humm though the sound system, filters didn't work.
The Far Oulet charge up real fast and has the charger built in too. We plug everything, sound system, keyboards, sound modules and electric guitars, and and left everything running for about eight hours before it died. Very good product
Funny story,
The very first Far Outlet we bought flew out the back of a bus, because someone left in the bus and the bus driver hit the gas and it flew out of the back of the bus knocking opening the door, the Far Outlet breaking into a million pieces on Dunlap Ave. in Phoenix and it was the first time we used at a away game.
And some freshmen, found what left of the a
nearly fully charged battery, took into the band room and wired up both +and - wires together and stated a electrical fire in the band room. The electric smell lasted for months in the band room.
I don't remember seeing the freshman back in band after that, I'm sure he did some time in detention after that.
The band director order a new Far Outlet and this time kept an eye on it.